1. Field of Invention
The invention relates to a fuel cell stack with a plurality of fuel cell elements which are layered on one another with separating plates located in between. At least one inside supply channel is provided to supply a combustion gas and at least one inside discharge channel is provided to discharge an exhaust gas, the channels extending in the stacking direction of the fuel cell elements.
2. Description of Related Art
Fuel cell stacks are used since a single fuel cell element produces only a very low voltage. In order to produce a voltage which can be used for application purposes, therefore several fuel cell elements are connected in series so that the cell voltages are added to one another. The fuel cell elements are arranged on one another such that, between the fuel cell elements and the separating plates, a respective intermediate space, on one side of the fuel cell element, a combustion gas and on the other side of the fuel cell element, an oxidizer being made available. The intermediate spaces for the combustion gas and the oxidizer are conventionally made in the form of several channels so that, between the channels, there is positive electrical contact between the fuel cell elements and the separating plates. In this way, the heat and current produced in the fuel cells can be discharged.
The combustion gases for fuel cell elements are hydrogen or a hydrogen-containing gas which is, accordingly, critical with respect to handling. Hydrogen-containing gas which escapes as a result of a fault or leak would react uncontrollable, for example, with the atmospheric oxygen and result in at least damage to the fuel cell stack. Therefore, the use of internal supply and discharge channels is known. For this purpose, in the individual fuel cell elements and the separating plates located in between, there are recesses which form channels in the assembled state of the fuel cell stack. Around the recesses are seals so that, with the corresponding bracing of the fuel cell stack, a tight channel is formed. The required tightness can be better ensured in this way than in external combustion gas supply.
A. J. Appleby; Fuel Cell Handbook, Van Nostrand Reinhold, New York, 1989 on pages 450 ff., discloses different versions of the supply of combustion gas and oxidizers. In a first version, there are guides for the combustion gas and the oxidizer such that the directions of the gas flows cross. The gas guides are open on the respective sides of the fuel cell stack, the respective gas flowing against the sides of the fuel cell stack. Fuel cell stacks in this so-called cross-flow technology, however, have a comparatively poor power density. Moreover, the external supply of combustion gas is problematic with respect to tightness and the unintentional escape of hydrogen-containing combustion gas.
In the second version shown, the combustion gas is routed via internal supply channels to the respective fuel cell elements. The oxidizer is supplied externally and is routed along the side of the fuel cell element which is the other one at the time in the transverse direction to the flow direction of the combustion gas.
A third version shows how the combustion gas and the oxidizer can be supplied so that a parallel flow direction of the two gases arises. This co-current technology or principle which is called counter-flow technology for the opposite flow direction has the advantage that the temperature distribution and the gas concentration are more uniform. The disadvantage is that a large number of feed channels and discharge channels must be provided; this results in a large number of seals and the associated tightness problems. Moreover, outside of the fuel cell stack, the effort for supply and discharge of the gases to the supply and discharge channels is very high; this makes fuel cell systems with these fuel cell stacks comparatively expensive. Also, the internal feed of oxidizers is disadvantageous because a high pressure loss occurs due to the complicated line routing, and thus, the oxidizer flow rate is limited. For compensation purposes, there can be stronger fans; however, this results in additional costs. In addition, the efficiency of the overall system deteriorates since increased driving power is necessary for the stronger fans.
External feed of the oxidizer in combination with co-current or countercurrent technology has not been feasible to date since, as a result of the feed and discharge channels for the combustion gas, there are too many components in the flow path, and therefore, a sufficient oxidizer flow rate cannot be achieved.
The limited oxidizer flow rate, in particular, has the disadvantage that the heat which is formed in the fuel cells is not adequately dissipated by the oxidizer, for example, air. The lower the flow rate of the oxidizer or air, the greater the danger of overheating of the fuel cell stack.
Another disadvantage in the known fuel cell stacks in co-current technology is that, as a result of the numerous supply and discharge channels, a large number of braces for the stack is necessary in order to ensure the required tightness. In this way, the fuel cell stack becomes very massive; this means increased construction effort, and thus, increased costs.